The application of various treatment and pretreatment solutions to metals to retard or inhibit corrosion is well established. This is particularly true in the area of metal food and beverage cans. Coatings are applied to the interior of such containers to prevent the contents from contacting the metal of the container. Contact between the metal and the food or beverage can lead to corrosion of the metal container, which can then contaminate the food or beverage. This is particularly true when the contents of the can are acidic in nature, such as tomato-based products and soft drinks. The coatings applied to the interior of food and beverage cans also helps prevent corrosion in the head space of the cans, which is the area between the fill line of the food product and the can lid; corrosion in the head space is particularly problematic with food products having a high salt content.
In addition to corrosion protection, coatings for food and beverage cans should be non-toxic, and should not adversely affect the taste of the food or beverage in the can. Resistance to “popping”, “blushing” and/or “blistering” is also desired.
Certain coatings are particularly applicable for application onto coiled metal stock, such as the coiled metal stock from which the ends of cans are made, “can end stock”. Since coatings designed for use on can end stock are applied prior to the ends being cut and stamped out of the coiled metal stock, they are also typically flexible and/or extensible. For example, can end stock is typically coated on both sides. Thereafter, the coated metal stock is punched, scored for the “pop-top” opening and the pop-top ring is then attached with a pin that is separately fabricated. The end is then attached to the can body by an edge rolling process. Accordingly, the coating applied to the can end stock typically has a certain degree of toughness and flexibility, such that it can withstand extensive fabrication processes, in addition to some or all of the other desirable features discussed above.
Various epoxy-based coatings and polyvinyl chloride-based coatings have been used in the past to coat the interior of metal cans to prevent corrosion. The recycling of materials containing polyvinyl chloride or related halide-containing vinyl polymers can generate toxic by-products, however; moreover, these polymers are typically formulated with epoxy-functional plasticizers. In addition, epoxy-based coatings are prepared from monomers such as bisphenol A (“BPA”), bisphenol F (“BPF”) and diglycidyl ethers of these (i.e. bisphenol A diglycidylether (“BADGE”) and bisphenol F diglycidyl ether (“BFDGE”); certain of these compounds have been reported as having negative health effects. While attempts have been made to scavenge the residual unreacted epoxy with, for example, acid functional polymers, this does not adequately address the problem; some free BADGE/BFDGE or their by-products will still remain. Government authorities, particularly in Europe, are restrictive on the amount of free BPA, BPF, BADGE, BFDGE and/or their by-products that are acceptable. Many compositions that meet these requirements, however, can have poor stability, such as polyesters that readily degrade in the presence of acid catalysts. Thus, there is a need for food and beverage can liners that are substantially free from BPA, BPF, BADGE, BFDGE and/or halogenated vinyl products; such can liners having resistance to degradation in acidic media are particularly desired.